Fair Value of Financial Instruments - Fair value, assets and liabilities measured on recurring and nonrecurring basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair value measurements, recurring
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Mortgage loans	$ 67,321	$ 568,480
Fair value measurements, recurring | Level 1, Quoted Prices in Active Markets
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Mortgage loans	0	0
Fair value measurements, recurring | Level 2, Observable Inputs Other Than Level 1 Prices
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Mortgage loans	0	0
Fair value measurements, recurring | Level 3, Unobservable Inputs
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Mortgage loans	67,321	568,480
Fair value measurements, nonrecurring
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Repurchase and loan agreements	1,100,106	1,220,972
Other secured borrowings		144,099
Fair value measurements, nonrecurring | Level 1, Quoted Prices in Active Markets
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Repurchase and loan agreements	0	0
Other secured borrowings		0
Fair value measurements, nonrecurring | Level 2, Observable Inputs Other Than Level 1 Prices
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Repurchase and loan agreements	1,105,473	1,226,972
Other secured borrowings		144,971
Fair value measurements, nonrecurring | Level 3, Unobservable Inputs
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Repurchase and loan agreements	$ 0	0
Other secured borrowings		$ 0